Method and system of monitoring respiratory signal by radio

ABSTRACT

A method and system is provided for monitoring a respiratory signal by radio. The method includes the steps of converting a change in electric resistance, which is caused by a change in abdominal circumference measured through a rubber waistband that is made of conductive rubber and is mounted on a lower garment of a testee during respiration, into a voltage signal, performing A/D conversion on the voltage signal, and transmitting the converted digital signal a short distance by radio using a wireless communication protocol for ZigBee, and receiving the respiratory signal transmitted by radio, transmitting it to a computer unit by wire through an RS-232 port that is a serial communication port, and enabling a tester to monitor the respiratory signal through a screen.

PRIORITY CLAIM

This application is a divisional of U.S. patent application Ser. No.11/879,999, filed on Jul. 19, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method and system ofmonitoring a biological signal by radio. More particularly, the presentinvention relates to a method and system of monitoring a respiratorysignal by radio, in which a respiratory frequency and a lung volume areprecisely measured using an elastic device fastened around the abdomenwithout the trouble of measuring the biological signal, the respiratorysignal, which is most frequently measured for inpatients, through anoral cavity.

2. Description of the Prior Art

In general, respiration is a physiological function that supplies freshair (oxygen) into the body and then releases a byproduct, carbondioxide, of the metabolism out of the body, and thus is essential forlife. The respiration, blood pressure, pulse, and body temperature areimportant biological signals showing a vital sign, and thus are thehighest measurement frequency of biological signals that must bemeasured three or four times for all the inpatients of the hospital fromday to day. For this reason, whether or not the respiration occurs ormeasuring and monitoring an amount of respiration is very importantmedically.

Up to now, respiratory airflow transducer, respiratory inductiveplethysmography, contactless respiration measurement, and breathing airtemperature measurement have been used or studied for sensing,measuring, and monitoring of a breathing signal.

As illustrated in FIG. 1 a, the respiratory airflow transducer convertsan amount of air, which is inhaled when a testee closes the nose tobreathe through the mouth with a breathing pipe 11 held in the mouth,into an electrical variable through a flow sensor 12 connected with thebreathing pipe 11, and measures an amount of respiration using theelectrical variable converted by the flow sensor 12. However, therespiratory airflow transducer is troublesome because the testee mustbreathe with the breathing pipe 11 held in the mouth. As such, therespiratory airflow transducer is used for a clinical spirometry testthat must continuously measure respiratory airflow with precision.

As illustrated in FIG. 1 b, the respiratory inductive plethysmography isa technique of measuring a change of the skin without the trouble ofholding the breathing pipe in the mouth of a testee, thereby estimatinga lung volume. In other words, the lung volume is estimated bycontraction and expansion of the lung. More specifically, the lungvolume is estimated by measuring and summing up changes of theperipheries of the thorax and abdomen caused by the respiration on thebasis of a principle that the respiration causes the volumes of thethorax and abdomen to be changed.

Elastic bands, in which thorax and abdomen coils 21 and 22 of conductivemetal are disposed in a zigzag shape, are fastened to the thorax andabdomen of the testee, respectively. As the peripheries of the thoraxand abdomen of the testee, to whom the thorax coil 21 and the abdomencoil 22 are attached breathes, are varied while the testee breathes, adistance between the adjacent crests (or roots) of each zigzag coil isvaried or displaced. Thereby, the inductances 23 of the thorax andabdomen coils that are attached to the thorax and the abdomen arechanged and measured electrically. At this time, although the lungvolumes are equal to each other, the contributions of the thorax and theabdomen to the lung volumes are dependent on the testee. Thus, therelative contributions k₁ and k₂ of each testee are calculated andapplied in advance.

However, the respiratory inductive plethysmography is difficult tohandle, and furthermore is impossible to wash with water, because theseparate elastic bands must be fastened on the clothes and because themetal coils are attached in the elastic bands. Further, because the ACsignal is required to measure the change of the inductance, a signalextracting circuit, which includes circuits of generating and measuringthe AC signal having constant frequency and amplitude, becomescomplicated.

As illustrated in FIG. 1 c, the contactless respiration measurement is atechnique for detecting respiration with no contact between a device andthe body, and makes use of the fact that during respiration, the skin ofthe thorax moves backwards and forwards to undergo displacement. Morespecifically, a wave generator 31 generates waves such as ultrasonicwaves or electromagnetic waves, and then sends the waves to the front ofthe body. Thus, the waves are reflected from the body. At this time, awave detector 32 detects properties of the reflected waves, and comparesthe reflected waves with the incident waves. Thereby, the displacementcaused by the periodic motion of the physical skin is measured. However,the wave signals are greatly attenuated in the air, and thus becomeweak. As a result, a quality of measurement becomes very bad.Furthermore, if the waves are not accurately emitted to the front of thebody in the direction perpendicular to the front of the body, suchmeasurement is impossible. The wave generator and detector are difficultto produce in the technical aspect, and thus have high production cost.Due to this problem, the contactless respiration measurement is underthe development, and thus has a small possibility of practical use.

As illustrated in FIG. 1 d, the breathing air temperature measurement isa technique based on the facts that, because a room temperature and abody temperature are is about 25° C. and about 37° C. respectively, adifference between the room temperature and the body temperature isabout 10° C., and that, because a temperature of expiratory air when atestee exhales is equal to the body temperature, it is higher than thatof inspiratory air when the testee inhales.

When a sensor (e.g. thermocouple or thermistor) 41 for sensing atemperature is located near the nostrils of the testee, a period oftemperature change is equal to a respiratory period, so that it can becalculated to measure a respiratory frequency. However, this techniquecan measure only the respiratory frequency, but not a variable relatedto ventilation (i.e. the volume of air breathed in and out of the lungs)such as a lung volume. According to the normal physiological function ofthe body, the lung volume is increased first when the metabolism of thebody increases to require increasing the ventilation, and then therespiratory frequency is increased only for still greater ventilation.Taking this fact into consideration, the measurement of the respiratoryfrequency makes it possible to determine whether or not the respirationoccurs, but it makes it impossible to measure the ventilation that ismore important than the respiratory frequency from the physiologicalviewpoint.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the prior art, and a first object of thepresent invention is to provide a method of monitoring a respiratorysignal by radio, in which the respiratory signal is monitored using anelastic waistband made of conductive rubber is fastened around theabdomen of a testee without the trouble of holding the breathing pipe inthe mouth of the testee, thereby providing a personal customizedrespiratory measurement technique that accurately measures a respiratoryfrequency and measures a lung volume within an allowable error range,and transmitting and monitoring the measured respiratory signal byradio.

A second object of the present invention is to provide a system foraccomplishing the first object.

To accomplish these objects, according to one aspect of the presentinvention, there is provided a method of monitoring a respiratory signalby radio. The method comprises: a respiratory signal measuring andtransmitting step of converting a change in electric resistance, whichis caused by a change in abdominal circumference measured through arubber waistband that is made of conductive rubber and is mounted on alower garment of a testee during respiration, into a voltage signal,performing analog/digital (A/D) conversion on the voltage signal, andtransmitting the converted digital signal to a short distance by radiousing a wireless communication protocol for ZigBee; and a respiratorysignal receiving and monitoring step of receiving the respiratory signaltransmitted by radio, transmitting it to a computer unit by wire throughan RS-232 port that is a serial communication port, and enabling atester to monitor the respiratory signal through a screen.

According to another aspect of the present invention, there is provideda system of monitoring a respiratory signal by radio. The systemcomprises: a connector that is connected to opposite ends of a rubberwaistband, the rubber waistband being mounted on a lower garment of atestee and being made of rubber including conductive particles; a radiotransmitter that is electrically connected with the connector, convertsa change in electric resistance, which is caused by a change inabdominal circumference measured through the rubber waistband duringrespiration, into a voltage signal, performs analog/digital (A/D)conversion on the voltage signal, and transmits the converted digitalsignal to a short distance by radio using a wireless communicationprotocol for ZigBee; and a radio receiver that receives the respiratorysignal, transmitted by radio using the ZigBee wireless communicationprotocol, through a reception antenna, converts the respiratory signalinto serial information, and outputs the converted serial informationthrough a RS-232 port. The respiratory signal is monitored bycalculating the input respiratory signal and checkup parameters at theradio receiver, and outputting the calculated result to a monitorscreen.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description when taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 a is a conceptual view for explaining a respiratory airflowtransducer used for detecting, measuring, and monitoring a respiratorysignal;

FIG. 1 b is a conceptual view for explaining a respiratory inductiveplethysmogrpahy used for detecting, measuring, and monitoring arespiratory signal;

FIG. 1 c is a conceptual view for explaining contactless respirationmeasurement used for detecting, measuring, and monitoring a respiratorysignal;

FIG. 1 d is a conceptual view for explaining breathing air temperaturemeasurement used for detecting, measuring, and monitoring a respiratorysignal;

FIG. 2 is a conceptual view for explaining configuration and operationof a system of monitoring a respiratory signal by radio according to thepresent invention;

FIG. 3 is a view illustrating an embodiment in which a rubber waistbandfor the system of FIG. 2 is mounted on a lower garment worn by a testee

FIG. 4 is a graph illustrating a change in electric resistance measuredwhile elongating the rubber waistband of FIGS. 2 and 3 by 0.5 cm;

FIG. 5 is a block diagram for explaining the configuration and operationof a radio transmitter in a system of monitoring a respiratory signal byradio according to the present invention;

FIG. 6 is a block diagram for explaining the configuration and operationof a radio receiver in a system of monitoring a respiratory signal byradio according to the present invention.

FIG. 7 is a graph showing the result of a respiratory monitoring (changein output voltage based on respiration) test using a system ofmonitoring a respiratory signal by radio according to the presentinvention; and

FIG. 8 is a graph showing correlation between abdominal respiratorysignal and tidal volume of a particular testee in a CO₂ inhalation testperformed for respiratory monitoring in a patient personal customizedtype using a system of monitoring a respiratory signal by radioaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail.

FIG. 2 is a conceptual view for explaining configuration and operationof a system of monitoring a respiratory signal by radio according to thepresent invention. FIG. 3 is a view illustrating an embodiment in whicha rubber waistband for the system of FIG. 2 is mounted on a lowergarment worn by a testee.

As illustrated in FIG. 2, the system of monitoring a respiratory signalby radio according to the present invention includes a rubber waistband110, which is mounted in a lower garment worn by a testee 102. Therubber waistband 110 mounted on the trousers is made of conductiverubber having electrical conductivity, rather than ordinary rubberhaving electrical non-conductivity. Owing to the elasticity, the rubberwaistband 110 also functions as a belt for the waist.

While the testee respires, the abdomen of the testee is changed incircumference, and simultaneously the rubber waistband 110 is elongatedor contracted. Thereby, the rubber waistband 110 is changed in crosssection and length, and thus is changed in electric resistance. A radiotransmitter 120 connected to the rubber waistband 110 converts theelectric resistance change, which is input through the rubber waistband110, into an analog voltage signal, performs A/D conversion on thevoltage signal, and transmits the respiratory signal based on digitalinformation to a short distance using a wireless communication protocolfor ZigBee.

The radio transmitter 120 is put into a pocket of the lower or uppergarment of the testee in electrical connection with the rubber waistband110. The ZigBee wireless communication protocol applied to the radiotransmitter 120 conforms to the IEEE 802.15.4 wireless standard for datanetworks, operates in frequency bands 2.4 GHz, 868 MHz, and 915 MHz,uses direct-sequence spread spectrum (DSSS) coding, has a data transferrate from 20 kbps to 250 kbps.

The radio signal transmitted from the radio transmitter 120 is receivedby a radio receiver 130. The radio receiver 130 receives the respiratorysignal transmitted using the ZigBee wireless communication protocol, andtransmits it to a computer unit 140 such as a personal computer (PC) bywire through an RS-232 port that is a serial communication port, so thatit enables a tester to monitor the respiratory signal through a screen.

The respiratory signal transmitted to the computer unit 140 by wire iscontinuously displayed on the screen, and exhibits clinically importantcheckup parameters, such as a tidal volume, a respiration frequency, anda minute ventilation, at fixed periods.

The testee is guaranteed mobility within a predetermined distance inwhich transmission and reception are performed by radio, and therespiratory signal of the testee is transmitted and monitored by radiowithout a stop motion. Accordingly, the trouble of the testee is reducedduring the respiration checkup.

As illustrated in FIG. 3, among the components of the present invention,the rubber waistband used to sense the abdominal circumference change ismade of conductive rubber, is mounted on the lower garment, which istypically worn by an inpatient, and also serves as the belt for thewaist. In the present invention, the opposite ends of the rubberwaistband 110 are connected to a connector 112 that can be connectedusing an electric wire. The connector 112 is electrically connected withthe radio transmitter 120, which is adapted to be carried in the pocket121 of the lower or upper garment of the testee. Thus, the rubberwaistband of the lower garment of the testee functions as a respiratorysensor by itself without mounting a separate sensor unit for measuringthe respiratory signal.

The conductive rubber used for the rubber waistband 110 of the presentinvention is produced by mixing a small quantity of conductive particles(e.g. of carbon, platinum, etc.) when a rubber as a nonconductor isformed in a desired shape. In this case in which the conductive rubberis produced so as to have the range from tens of Ω to several Ωaccording to a mixing ratio, the electric resistance change caused bythe change in length can be easily measured.

FIG. 4 is a graph illustrating a change in electric resistance measuredwhenever the rubber waistband of FIGS. 2 and 3 is elongated by 0.5 cm.FIG. 5 is a block diagram for explaining the configuration and operationof a radio transmitter in a system of monitoring a respiratory signal byradio according to the present invention. Further, FIG. 6 is a blockdiagram for explaining the configuration and operation of a radioreceiver in a system of monitoring a respiratory signal by radioaccording to the present invention.

As illustrated in FIG. 4, the rubber waistband 110 of the presentinvention is cut to a proper length so as to be mounted on the lowergarment, and then the change of electric resistance is measured wheneverthe cut rubber waistband 110 is gradually elongated by 0.5 cm. As aresult, as the length of the rubber waistband 110 increases, a value ofthe electric resistance is increased exponentially.

In this manner, when the length is changed within a range of 1 cm orless (range from 56 cm to 57 cm in FIG. 4), the electric resistanceshows linear correlation in which it is almost proportional to thelength. When the respiratory signal is measured within this range, anoptimal sensitivity of measurement can be obtained. In other words,while the abdominal circumference of the testee is periodically changedby the respiration of the testee, the electric resistance of the rubberwaistband 110 mounted on the lower garment of the testee is alternatelyincreased and decreased.

The electric resistance change measured through the rubber waistband 110is input into the radio transmitter 120. As illustrated in FIG. 5, theradio transmitter 120 establishes a circuit such that the electricresistance of the rubber waistband 110 operates to have the resistancein one arm (of four arms) of the Wheatstone bridge circuit 122. It isassumed that the electric resistance of the expanded rubber waistband110 when the testee exhales is R. The rubber waistband 110 is connectedwith three resistors having the same resistance as the electricresistance thereof, and thereby the bridge circuit is constructed. Atthis time, voltages V₊ and V⁻ on the bridge circuit are expressed by thefollowing Equation 1.

$\begin{matrix}{V_{+} = {V_{-} = \frac{V_{E}}{2}}} & (1)\end{matrix}$

In Equation 1, V_(E) is the DC voltage that drives the bridge circuit.

When the testee begins to inhale, the electric resistance increases toR+ΔR, and thus the voltage V+ of Equation 1 is transformed into thefollowing Equation 2.

$\begin{matrix}{V_{+} = {{\frac{R + {\Delta \; R}}{R + \left( {R + {\Delta \; R}} \right)}V_{E}} = {\frac{1 + \frac{\Delta \; R}{R}}{1 + 1 + \frac{\Delta \; R}{R}}V_{E}}}} & (2)\end{matrix}$

In Equation 2, if the change ΔR of the electric resistance that isincreased by inhalation is sufficiently less than R (i.e. R>>ΔR), ΔR/Rof the denominator is approximated to 0, and thus Equation 2 istransformed into the following Equation 3.

$\begin{matrix}{{V_{+} \approx {\frac{1 + \frac{\Delta \; R}{R}}{2}V_{E}}} = {\frac{V_{E}}{2} + {\frac{\Delta \; R}{2R}V_{E}}}} & (3)\end{matrix}$

In Equation 3, because the voltage V⁻ maintains the value, V_(E)/2, ofEquation 1 irrespective of the elongation of the conductive rubberwaistband (FIG. 5), a difference between V₊ and V⁻ is calculated, andthereby is expressed by the following Equation 4.

$\begin{matrix}{{V_{+} - V_{-}} = {\frac{\Delta \; R}{2R}V_{E}}} & (4)\end{matrix}$

Accordingly, the voltage difference between V₊ and V⁻ is proportional tothe electric resistance change ΔR of the conductive rubber waistband. Inother words, when the DC Wheatstone bridge circuit 122 of FIG. 5 isused, the electric resistance change is converted into a voltage signal.The voltage signal output from the Wheatstone bridge circuit 122 isapplied to a differential amplifier circuit 124, and then thedifferential amplifier circuit 124 amplifies and outputs a differencebetween the input V₊ and V_voltage signals to one voltage signal.

The voltage signal amplified by the differential amplifier circuit 124is applied to a low-pass filter circuit 125, and then the low-passfilter circuit 125 extracts a voltage signal corresponding to therespiratory signal, from which high-band noise is minimized by allowingonly a low-band signal to pass therethrough.

The respiratory signal output from the low-pass filter circuit 125 isinput into an A/D converter circuit 126, and thereby is converted into adigital signal. In other words, because the respiratory signal outputfrom the low-pass filter circuit 125 is an analog voltage signal, it isconverted into a digital signal (information) by the A/D convertercircuit 126.

The digital signal output from the A/D converter circuit 126 is inputinto a ZigBee transmission circuit 127, and then is transmitted to ashort distance through a transmission antenna 128 by radio. The A/Dconverter circuit and the ZigBee transmission circuit can be easilyrealized using a commercialized semiconductor chip. Since the entireradio transmitter 120 can be configured of a low electric power circuit,it can be operated with a dry battery, and be reduced in size whenproduced. As a result, the radio transmitter is designed to be carriedin a pocket of a lower or upper garment of the testee. In this manner,because the radio transmitter 120 is carried in the pocket, therespiratory signal of the testee can be freely monitored during movingwithin a range in which it can be received by radio.

As illustrated in FIG. 6, the respiratory signal, which is transmittedfrom the ZigBee transmission circuit 127 of the radio transmitter byradio, is input into a ZigBee reception circuit 132 of the radioreceiver 130 through a reception antenna 131, is converted into serialinformation, and is input into the computer unit 133 through the RS-232port. The computer unit 133 functions to display the respiratory signaland the whole checkup parameters on the monitor screen 134, and toperform respiratory monitoring for which a user makes a request.

FIG. 7 is a graph showing the result of a respiratory monitoring test(change of output voltage based on respiration) using a system ofmonitoring a respiratory signal by radio according to the presentinvention. FIG. 8 is a graph showing correlation between abdominalrespiratory signal and tidal volume of a particular testee in a CO₂inhalation test performed for respiratory monitoring in a patientpersonal customized type using a system of monitoring a respiratorysignal by radio according to the present invention.

FIG. 7 shows the test result using the system of monitoring arespiratory signal by radio according to the present invention. Thetestee sequentially performed comfortable respiration, maximuminhalation, comfortable respiration, unnatural cough, and maximuminhalation in the state where he/she sited on a chair in the lowergarment on which the conductive rubber waistband was mounted. At thattime, the respiratory signals were monitored by radio.

In FIG. 7, the vertical axis represents the time, and the horizontalaxis represents the voltage. It can be found that accurate respiratoryfrequencies were obtained because respective respiratory modes(patterns) were easily discriminated by obtained respiratory signals,and because periodical respiration was accurately recognized. Becausethe possibilities of measuring the accurate respiratory frequencies andsimultaneously discriminating the respiratory modes were proved as shownin FIG. 7, a measurement test of the lung volume, as shown in FIG. 8,was performed in a manner such that, in order to increase a tidal volumewithout being unconscious of his/her respiration in a normal state, thetestee breathed a mixed gas consisting of carbon dioxide from 0% to 5%and air through the mouth for three minutes, and the respiratory signalswere monitored for one minute when reaching a steady state

The testee held a respiratory airflow transducer in his/her mouth, andthen accurate respiratory airflow and a tidal volume were measuredtogether with an abdominal respiratory signal. It can be seen from thecorrelation between the abdominal respiratory signal and the tidalvolume as shown in FIG. 8 that the abdominal respiratory signalincreased when the tidal volume increased by CO₂ inhalation, and wasstatistically significant (P<0.005) because a coefficient of correlationwas over 0.96.

Therefore, the fact that the system of monitoring a respiratory signalby radio according to the present invention could estimate the tidalvolume in a relatively accurate manner by monitoring the respiratorysignal from the conductive rubber waistband 110 by radio wasexperimentally proved.

As described above, when the abdominal respiratory signals are monitoredusing the conductive rubber waistband (or belt), the respiratoryfrequencies can be accurately measured without an error as in the graphof FIG. 7, and the tidal volume has linear correlation with respect toan actual true value within a relative error from 10% to 20%.

This relative error is attributed to a relative ratio at which therespiration varies the volumes of the thorax and abdomen and isdistributed to the thorax and the abdomen. As such, the contribution ofthe thorax is regarded as nothing in the present invention because onlythe tidal volume of the abdomen is measured. Although the contributionsof the thorax and abdomen are different from each other depending on thetestees, they are invariably maintained to each particular testee. Thus,the tidal volume can be reliably measured even when it is measuredrespect to only the abdomen. In other words, after each testee issubjected to customized calibration in terms of an individualdifference, the respiratory signals are measured. Thereby, accuratemeasurement results can be obtained.

The system of monitoring a respiratory signal by radio according to thepresent invention is designed so that the elastic conductive rubberwaistband (or belt) serving as the respiratory sensor is mounted on thelower garment of the patient. The lower garments are not exchangedbetween the patients, so that the personal customized calibration iseffective. As shown in FIG. 8, in order to carry out the personalcustomized calibration, a calibration procedure of performing the CO₂inhalation test on each testee once, obtaining the correlation betweenthe abdominal respiratory signal and the tidal volume of the particulartestee, calculating a regression line, which obtains the lung volumefrom the abdominal respiratory signal, from the correlation, and thensubstituting the abdominal respiratory signal into the regression lineto measure the lung volume has only to be performed in advance.

As can be seen from the foregoing, the present invention improvesconvenience of use because it does not provide the trouble of mounting aseparate unit in the mouth or nose when the respiratory signals ismeasured and checked up, can be cleaned due to the conductive rubbermaterial unlike a complete conductor, and can be the garment wearabletype serving as the belt for the waist. Further, because the DC is usedinstead of the AC when the signal is extracted, the circuit issimplified. Due to the radio transmission mode, the measurement isperformed in a free state without restricting activity of the patient.Furthermore, the respiratory monitoring is possible during moving. Inaddition, the accurate lung volume can be estimated only by the abdomenrespiratory measurement through the personal customized calibration.

Although a preferred embodiment of the present invention has beendescribed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A system of monitoring a respiratory signal by radio, the systemcomprising: a connector that is connected to opposite ends of a rubberwaistband, the rubber waistband being mounted on a lower garment of atestee and being made of rubber including conductive particles; a radiotransmitter that is electrically connected with the connector, convertsa change in electric resistance, which is caused by a change inabdominal circumference measured through the rubber waistband duringrespiration, into a voltage signal, performs analog/digital (A/D)conversion on the voltage signal, and transmits the converted digitalsignal to a short distance by radio using a wireless communicationprotocol for ZigBee; and a radio receiver that receives the respiratorysignal, transmitted by radio using the ZigBee wireless communicationprotocol, through a reception antenna, converts the respiratory signalinto serial information, and outputs the converted serial informationthrough a RS-232 port, wherein the respiratory signal is monitored bycalculating the input respiratory signal and checkup parameters at theradio receiver, and outputting the calculated result to a monitorscreen.
 2. The system as claimed in claim 1, wherein the radiotransmitter comprises: a Wheatstone bridge circuit, one resistor ofwhich operates the rubber waistband to convert a change in electricresistance of the rubber waistband into a voltage signal; a differentialamplifier circuit that amplifies and outputs a difference between thevoltage signals output from the Weston bridge circuit to one voltagesignal; a low-pass filter circuit that extracts a voltage signalcorresponding to the respiratory signal, from which high-band noise isminimized by allowing only a low-band signal to pass therethrough, fromthe voltage signal amplified through the differential amplifier; ananalog/digital (A/D) converter circuit that converts the respiratorysignal output from the low-pass filter circuit into a digital signal;and a ZigBee transmission circuit that transmits the digital signaloutput from the A/D converter circuit through a reception antenna byradio to a short distance using a ZigBee wireless communicationprotocol.
 3. The system as claimed in claim 1, wherein the radiotransmitter is carried in a pocket of a lower or upper garment of thetestee.
 4. The system as claimed in claim 1, wherein the A/D convertercircuit and ZigBee transmission circuit of the radio transmitter makeuse of a semiconductor ship.